3 UK is the only mobile phone operator in the UK to offer free Skype to Skype calls.
We're not sure whether they will actually allow Skype calls to be made over the iPad but given the fact that Skype has already released an iPad friendly client since early April, this is only a matter of time before they allow such a thing.
As it stands, you should be able to buy an iPad, downsize a PAYG 3 UK SIM card, put £10 worth of credit on it and start making so calls after having downloaded and installed the Skype client (opens in the App Store).
Arguably, the iPad comes with Bluetooth 2.1 which means that you will be able to make calls on it using a wireless Bluetooth headset like this Plantronics Discovery headset. Calls in the UK on Skype costs £3.99 per month for unlimited landline calls and £13.49 for 120 minutes to UK mobiles.
Unfortunately, it seems that Skype on the iPad won't support Voice dialling for the time being and hopefully that will change in a near future. 3 UK also gives 150MB of data allowance to its PAYG users each time they top up.